This invention relates to a method and apparatus for growing mercuric iodide crystals, particularly to growing such crystals in a horizontal furnace, and more particularly an improved horizontal furnace growing method which utilizes controlled, movable radial and axial airflows for maintaining a desired temperature gradient during crystal growth.
Mercuric iodide (HgI.sub.2) has shown good prospects as a high-Z material for use as a room-temperature nuclear radiation detector because of its large band gap (.about. 2.1 eV). Single crystals of HgI.sub.2 can be grown by various methods (see M. Schieber et al., J. of Crystal Growth, 24/25, 205, 1974), but experience has shown that crystals grown from the vapor phase are preferred for fabricating detectors, although very thin sections of solution-grown crystals can also be used. Crystals can be grown from the vapor phase in a system with a static temperature profile or by means of a temperature oscillation method.
The temperature oscillation method (TOM), which was initially developed by H. Scholz and co-workers (see Acta Electronic, 17, 69, 1974; Chem. Ing. Tech., 37, 1173, 1965; Crystal Growth, Oxford, Ed. H. S. Peiser, Pergammon, 475, 1967; and Philips Tech. Rev., 28, 316-319, 1967), consists of a periodic reversal of the temperature gradient between the source material and the crystal which causes alternating crystal growth and reevaporation or redissolution. The TOM has been successfully applied to the growth of HgI.sub.2 from the vapor by H. Scholz (see Philips Tech. Rev. above), and was also used to grow GaP, .gamma.Fe.sub.2 O.sub.3, and NiFe.sub.2 O.sub.4 by chemical vapor transport, CuS and CoS.sub.2 by vapor transport, and Se by hydrothermal growth. Crystals of yttrium phosphate, arsenate, and vanadate were grown from the flux by superimposing periodic temperature oscillations on a linear cooling curve. The TOM has been mainly applied to the growth of HgI.sub.2 in a vertical furnace which has both an axial and a radial gradient, and in a horizontal furnace which has only an axial gradient. The crystals grown in a vertical furnace were very large (up to 100-gram weight) but showed growth rings which represent inhomogeneities both in dislocation density and in impurity distribution. The crystals grown in a horizontal furnace (such as described and claimed in co-pending U.S. Patent application Ser. No. 592,481, filed July 1, 1975, in the name of M. M. Schieber et al.), being only an axial gradient, are small (up to 5-gram weight), but are free of growth inhomogeneities. Furthermore, it appears that there is a larger percentage of "good" crystals grown in a horizontal rather than in the vertical furnace, a "good" crystal being defined as one which has a mobility (.mu.) times trapping time (.tau.), .mu..tau. product for holes larger than 10.sup.-6 cm.sup.2 /V. Thus, a need exists in the prior art for an HgI.sub.2 crystal furnace which provides the advantages of each of the prior known vertical and horizontal furnaces without their above-mentioned associated disadvantages.